The use of electrochemical means of detection has often been chosen for its simplicity, both in terms of device manufacture and in terms of ease of use. The principle mode of selectivity of electrochemistry (both for amperometric and potentiometric modes) is the reduction-oxidation (also called “redox”) potential of the analyte (which is the chemical species of electrochemical interest). For example, using the technique of amperometry (where the potential is applied to the electrode, and the resulting current is measured), the selectivity towards the analyte is achieved based on the redox potential of the analyte.
The signal that is generated at the electrode can depend on many factors and properties of the electrochemical system. Examples of properties of the sample that affect the transport of the analyte include viscosity, temperature, density, and ionic strength. The variations that affect the transport of the analyte can subsequently affect the measured electrochemical signal. Examples of such transport mechanisms include diffusion, migration, and convection.
In another example, the properties of the electrode itself can affect the transport of the analytes and/or the kinetics of any reactions that may generate the measured electrochemical signals. Examples of such properties include the effective electrode area, the geometry of the electrodes, the geometry of the sample chamber, the extent of electrode fouling, diffusional barrier membranes over the electrode, and catalytic properties of the electrode material.
Electrochemical sensors are commonly found in a number of sensing applications, from medical biosensors to environmental and gas sensors. There are commonly two modes of electrochemical measurement, amperometric and potentiometric. Amperometric sensors operate on the principle of applying a voltage potential to an electrode and measuring the resulting current. Examples of amperometric sensors include most commercial glucose biosensors and many gas sensors. Potentiometric sensors operate on the principle of applying a current to an electrode and measuring the resulting potential. It is often the case that the applied current is kept at zero amps. The pH electrode is an example of a potentiometric sensor.
FIG. 1 shows the action of an amperometric sensor in which a voltage is applied to the electrode 310 which causes a particular analyte (the substance being measured) in the sample to be oxidized (i.e., giving up electrons to the electrode). The oxidation causes a current 315 to be generated which can then be detected and analyzed. The potential at which the analyte oxidizes is called the “oxidation potential” of the analyte.
Generally speaking, the term “redox potential” is used to indicate the potential at which an analyte is either oxidized or reduced. In the sensor of FIG. 1, ferrocyanide (“FERRO”) 300 transfers electrons to the electrode if the potential is high enough to cause the electrochemical reaction to occur. Once the electrons are transferred, ferrocyanide is oxidized to ferricyanide (“FERRI”) 305.
Thus, in FIG. 1, a sufficiently high potential is being applied to oxidize ferrocyanide, the reduced form of the electroactive species, to the oxidized form, ferricyanide, and the resultant current 315 detected by the electrode depends on the concentration of the reduced species.
As discussed above, the current from amperometric sensors depends on a number of factors in addition to the concentration of the analyte of interest. Traditional amperometric methods rely on the assumption that only the concentration of the analyte changes from measurement to measurement; hence, when other factors of the electrochemical system vary, the measured signal and the estimate of the analyte concentration can be incorrect. Potentiometric sensors also suffer from related factors, including transport of the analyte and electrode fouling. Variations in these factors would add uncertainty and error to the measured signal. For example, FIG. 2 shows the DC current from two amperometric sensors where the effective electrode area is changed. Data points 455 are measured in a sample containing 10 mM ferrocyanide. Data points 450 are measured in a sample containing 20 mM ferrocyanide. In both cases, as the electrode area varies, the measured DC current signal varies as well. Furthermore, for a given electrode area, increasing the analyte concentration from 10 mM to 20 mM results in measuring an increased current signal. This illustrates the dependence of the measured DC current signal on the electrode area and on the analyte concentration.
Several factors may contribute to a sensor having variable electrode area. One source may be errors during manufacturing that may lead to variability in the electrode area from sensor to sensor. Another factor may be deterioration of the electrode during use. Another factor may be incomplete contact of the sample with the sensor electrode, examples of which are illustrated in FIGS. 8 and 9.
FIGS. 8a through 8c are schematic diagrams of a typical electrochemical test strip that forms the basis for many commercially available glucose biosensors. In FIG. 8a, there are two electrodes 355, each of which is connected to leads 350 that interface with the electronics of the meter. The electrodes 355 and leads 350 may be coupled to a support substrate 375. In this example, the test strip uses a commonly used 2-electrode configuration. In FIG. 8a, the sample 360 completely covers both electrodes, ensuring that the entire electrode area of each electrode is in contact with the sample. In FIG. 8b, sample 370 covers one electrode completely but partially covers the other electrode. In FIG. 8c, sample 365 partially covers both electrodes.
FIG. 9 illustrates partial coverage of electrodes by a sample for a different geometry of electrodes. In this example, an electrochemical test strip is made with two electrodes facing each other in a parallel plate design. Electrode 400 and electrode 405 are supported by a solid substrate material 420. Sample 410 fills the sample chamber and covers both electrode areas fully. Sample 415, however, only partially covers both electrode areas and results in a system of reduced effective electrode area. Such incomplete coverage of the electrode surface can be a result of partial filling of the sample chamber. In one example, diabetic patients that make blood glucose measurements must often use such electrochemical test strips to make measurements of blood glucose. In such cases, if enough blood does not enter into the sample chamber, incomplete coverage of the electrode system can result, yielding inaccurate glucose estimates. Thus, a method to assess the effective electrode area that is independent of the analyte concentration would be useful.
Furthermore, the volume of sample that enters into the test strip can be estimated. Referring to FIG. 9, if the three dimensions of the sample chamber that contains sample 415 are known, then the volume of sample 415 can be estimated by scaling the total geometric volume of the sample chamber by the fractional amount of the electrode coverage. In one example, the total volume of the sample chamber is 100 nL. If sample 415 is determined to cover 75% of the electrode 405, then one estimate of the volume of sample 415 would be (0.75)*100 nL=75 nL. The estimate of the sample volume would be useful when making measurements that depend on knowing the volume of the sample in the electrochemical cell. One example of where this knowledge would be useful is in coulometry.
FIG. 4 illustrates the problem of electrode fouling with electrochemical sensors. Electrode fouling, also called sensor fouling, is a term that describes material 320 adhering, adsorbing, or otherwise coating all or part of the electrode 310. In this example, the analyte is ferrocyanide 300 which must move through the fouling material 320 and then react at the electrode 320 in an oxidation reaction that yields an electronic current 315 in the electrode 310. The product of the reaction is ferricyanide 305 which then moves back out of the fouling material 320. One example of when electrode fouling may occur is during extended use of the sensor in environments that could cause fouling, such as implanting a biosensor into the body or deploying gas sensors in environments containing sulfides. In such situations, as well as other situations that would be apparent to one skilled in the art, material may deposit onto the electrode, causing a distorted signal to be measured. Often, the measured signal intensity decreases as the amount of electrode fouling increases until ultimately the sensor becomes insensitive to the target analyte. In other cases, the fouling material may act as a catalyst for certain chemical reactions and the sensor's response may actually be enhanced. In either case, if the sensor's response is altered due to fouling, then the resulting measurement is inaccurate.
In the calibration curves shown in FIG. 3, data points 470 measure the DC current from samples with different concentrations of ferrocyanide with an electrode that is not fouled. Data points 480 measure the DC current from samples with different concentrations of ferrocyanide with an electrode that is fouled by a coating of 3.33 μg of cellulose acetate. Data points 490 measure the DC current from samples with different concentrations of ferrocyanide with an electrode that is fouled by a coating of 10 μg of cellulose acetate. This example illustrates that the measured DC current signal in this amperometric sensor depends on both the analyte concentration and the extent of electrode fouling. Thus, a low DC signal may be the result of either low analyte concentration or due to increased electrode fouling. Thus, a means to determine the extent of electrode fouling which is independent of analyte concentration would be useful. Such a method could then be used to adjust the measured current signal and correct for signal distortion caused by electrode fouling.
Although the previous two examples were illustrated with amperometric sensors, one of ordinary skill in the art will recognize the application to potentiometric sensors. Potentiometric sensors also rely on analyte coming into the proximity of the electrode.
Thus, when electrochemical means of detection are used, the environmental factors—including the properties of the sample that contain the analyte—may heavily influence the signal that is measured. Such factors may introduce inaccuracies into the measurement, including but not limited to, change in calibration and change in sensitivity. Hence a method and apparatus for detecting properties of the environment that may affect the measured signal, including dielectric constant of the sample or the electrode, effective electrode area, and ionic strength of the sample, would benefit electrochemical sensor systems and may allow for corrections to be made to the estimated analyte concentration, calculated from the measured signal, based on the information about the environmental factors.